The present invention relates generally to halogen-free, phosphorus-free flame-retardant epoxy resins suitable for use in making printed circuit boards and in semiconductor encapsulation applications.
Epoxy resins have the excellent characteristics of moisture, solvent and chemical resistance, toughness, low shrinkage on cure, superior electrical and mechanical resistance properties, and good adhesion to many substrates. The versatility in formulation also make epoxy resins widely applicable industrially for surface coatings, adhesive, painting materials, potting, composites, laminates, encapsulants for semiconductors, and insulating materials for electric devices, etc. o-Cresol formaldehyde novolac epoxy (CNE) is the resin typically employed in the encapsulation of microelectronic devices. Several approaches for modification of epoxy backbone for enhancing the thermal properties of epoxy resins have been reported. Aromatic bromine compounds in conjunction with antimony oxide are widely used as a flame retardant for epoxy resins. Tetrabromobisphenol A is a typical example of the aromatic bromine compounds used as a flame retardant for epoxy resins. An excess amount of epoxy resin is reacted with tetrabromobisphenol A to prepare an advanced epoxy resin having two terminal epoxide groups, as shown in the following formula: 
wherein EP is a bi-radical group of the backbone of the epoxy resin, and m is an integer of 1-10. The advanced epoxy resin can be used in preparing a flame-retardant printed circuit board (FR-4) by impregnating glass fibers with the advanced epoxy resin and heating the resulting composite to cure the advanced epoxy resin. Furthermore, the advanced epoxy resin can be employed to encapsulate microelectronic devices, in which the advanced epoxy resin is cured at a high temperature with a curing agent, so that an encapsulant having a flame-retardant property is formed.
Although the tetrabromobisphenol A-containing advanced epoxy resin shows flame retardant property, major problems encountered with this system are concerned with the generation of toxic and corrosive fumes during combustion such as dioxin and benzofuran.
The flame retardant having a small molecular weight tends to degrade the mechanical properties of the epoxy resins, and migrate/vaporize from the epoxy resins such that the flame retardancy thereof diminishes.
Owing to organic phosphorus compounds generate less toxic gas and smoke than halogen-containing compounds, some authors have reported advanced epoxy resins containing phosphorus compound, for examples U.S. Pat. Nos. 6,291,626; 6,291,627; and JP 2001-220427.
Although phosphorus-containing flame retardants can replace the conventional halogen-containing flame retardants in the aspect of flame retardancy, the phosphorus-containing flame retardants inevitably cause the eutrophication in rivers and lakes after hydrolysis, and create environmental problems. Further, their high absorbency of moisture leads to decomposition, and thus will jeopardizes the long time reliability of a semiconductor product. Therefore, there is a need in the semiconductor product encapsulation industry for developing a halogen-free, phosphorus-free flame-retardant epoxy resin.
The invention of the present application provide a halogen-free, phosphorus-free flame-retardant advanced epoxy resin, and epoxy composition. They are suitable for use in the fabrication of printed circuit boards and as an encapsulation material for semiconductor products.
The halogen-free, phosphorus-free flame-retardant advanced epoxy resin synthesized according to the present invention has the following formula (I): 
wherein Q is 
0 less than n less than 60; and
Ep is 
wherein 0 less than m less than 12; 0 less than p less than 12; R1 is hydrogen or methyl; and X=A or B, and at least one of X is B, wherein 
The halogen-free, phosphorus-free flame-retardant epoxy composition disclosed in the present invention comprises 10xcx9c50 wt % of an epoxy resin having two or more epoxide functionalities; 10xcx9c40 wt % of an inorganic filler; and 20xcx9c80 wt % of the advanced epoxy resin (I).
Preferably, Q in the formula (I) is methylene.